The present invention relates to a device and a method for monitoring scale, and particularly scale in a hydrocarbon well flow system.
The formation of scale is a well-known problem in the oil and gas industry. Scales can develop when (relatively high) formation fluid temperatures and pressures are reduced during fluid extraction, the temperature and pressure reduction encouraging the precipitation from solution of scale-forming species. The scale deposits can cause undesirable constrictions or blockages in well production tubing and in the well formations themselves.
Particularly problematic scale deposits are those containing barium salts, generally observed as BaSO4. These are much more difficult to remove than, for example, calcium carbonate scale. Thus it is desirable to be able to detect the build-up of barium scale as early as possible.
In addition to barium, formation water generally contains naturally occurring radium in solution leached from the formation. Different isotopic species of radium may be present in the formation as members of both the uranium (see FIG. 1) and thorium (see FIG. 2) radioactive decay series. As a relatively soluble member of the decay chains, radium is taken into solution in formation water and carried into the borehole. Because radium and barium chemistries are very similar, radium is co-precipitated when barite scale is deposited.
Therefore, scales often contain radioactive material. This means that contaminated completion equipment must be dealt with and disposed of according to radiation safety legislation. However, forewarning of scale deposition, and analysis of the properties of the deposit formed, allows appropriate prevention or removal strategies to be adopted and radiation safety procedures to be planned effectively.
U.S. Pat. No. 6,037,585 describes a method of locating downhole scale and determining the flux of gamma-rays emitted therefrom. This information is then used to correct downhole gamma-ray measurements so that activity due to artificially introduced tracer isotopes can be identified. The method uses wireline logging equipment incorporating a spectroscopic gamma-ray tool.
U.S. Pat. No. 5,038,033 describes another method of detecting downhole radioactive deposits and determining the level of radioactivity using wireline logging equipment.
U.S. Pat. No. 4,856,584 relates to a method of inhibiting scale formation in which a gamma-ray detector is used to monitor scale build up.
The present invention is at least partly based on the realisation that, because the radium isotopes in scale decay into characteristic sequences of daughter products, useful information concerning scale formation can be obtained by determining the abundances of radioactive isotopes in the scale. In particular, one of the daughter products of radium is radon which, being a gas, can escape from some types of scale and therefore have a significant effect on the decay sequences. Also the ratio of 228Ra to 226Ra significantly affects the character of the scale activity.
In the following, we understand xe2x80x9cabundancexe2x80x9d to be either relative abundance or absolute abundance. In practice relative abundancy values are generally easier to obtain, and in most of the aspects of the invention discussed below relative abundancy values are as acceptable as absolute values. However, where absolute values are desirable this is mentioned.
Also in the following, by a xe2x80x9cgamma-ray spectrumxe2x80x9d we mean a measure of the relative intensities or count rates of gamma-rays in a plurality of respective gamma-ray energy ranges. Preferably the spectrum has at least three, and more preferably at least five, discrete ranges. Clearly the more ranges there are in the spectrum, the more spectroscopic detail is revealable. In practice, however, the number of ranges is limited by e.g. the need to provide a robust detector and the need to provide sufficient data channels from the detector.
In a first aspect, the present invention provides a method of analysing scale at a location in a hydrocarbon well flow system, comprising the steps of:
(a) using an in situ gamma-ray detector to obtain a gamma-ray spectrum from the scale, and
(b) spectroscopically analysing the spectrum to determine the abundances of radioactive isotopes in the scale.
Preferably, the method further comprises the step of:
(c) repeating steps (a) to (b) to monitor the development of the scale.
Because the abundances are related to the amount and manner of deposition of the scale, determining the abundances and monitoring the development of the scale can provide useful information about the formation of the scale and the behaviour of the environment in which the scale is forming. For example, the relative abundances can be indicative of the chemistry of the fluid from which the scale deposits, and a change in the relative abundances may indicate a change in the chemistry of that fluid. So, if the scale deposits from production water, a change in the relative abundances may indicate an alteration in the relative amounts of sea and formation water in the production water.
Also obtaining the spectrum in situ, and repeating steps (a) and (b) allows continuous observation of the scale. The early stages of scale deposition can then be observed so that appropriate remedial action (e.g. use of scale dissolvers or inhibitors) can be taken before the scale thickens and becomes less responsive to such action. In contrast, conventional wireline logging techniques make only a single measurement of scale radioactivity as the logging tool passes along the well production tubing, so that continuous monitoring is not possible and the early stages of scale deposition are easily missed.
The method may further comprise the step of:
(d) using the abundances to determine the specific activity of the scale. An advantage of determining the specific activity in this way is that it is possible to compensate appropriately for e.g. different scale 228Ra to 226Ra ratios and the escape of radon from the scale. In contrast, if a conventional determination were made on the basis of only a total count rate (without compensation for e.g. the 228Ra to 226Ra ratio and radon loss), the specific activity could in some cases be over-estimated by more than an order of magnitude.
This can be significant because specific activity determinations are often used to help decide whether e.g. special radiation protection measures need to be adopted for the protection of personnel, and the disposal and/or decontamination of contaminated equipment. Such measures are generally costly and inconvenient, and so they are usually adopted only when necessary. Thus it is clearly desirable to have available the most accurate data possible. The method may further comprise the step of:
(e) using the abundances to determine the permeability of the scale. This makes use of the principle that the abundances are related to the proportion of radon which escapes from the scale, and the proportion of escaped radon is in turn related to the permeability and deposition rate of the scale.
Because different types of scale have characteristic permeabilities, this embodiment of the method can provide information about the type of scale which is being formed. An operator might then be in a better position e.g. to select an appropriate form of scale treatment.
The method may further comprise the step of:
(f) using the abundances to determine the amount of radium originally deposited in the scale, and
(g) deriving the quantity of scale from the amount of radium and from the relative concentrations of radium and the other scale components in the fluid from which the scale deposits. In this embodiment of the method more accurate determinations of the quantity of scale are generally obtained if, to the extent that is possible, absolute abundancies are determined at step (b).
Based on the amount of radium originally deposited in the scale, and the relative concentrations of radium and the other scale components (particularly Ba) in the fluid from which the scale deposits, it is then possible to derive the amount of scale. This is a significant improvement over conventional techniques for deriving the amount of scale based on the amount of radium presently located in the scale, because the method of this aspect of the invention takes account of e.g. decreases in radioactivity caused by radon escape from the scale. Without compensation for these decreases, erroneously low values for the amount of scale may be calculated.
Preferably the detector of the method is held stationary.
Preferably the detector is permanently or semi-permanently installed in the hydrocarbon well flow system, e.g. by being fixed to a well borehole externally of the production tubing. An advantage of permanent or semi-permanent installation is that deployment of the detector does not then interfere with the production of hydrocarbons from the well. This is in contrast to conventional wireline logging in which production has to stop as the logging tool passes through the production tubing.
Preferably, in step (a) the spectrum is obtained over a time interval of at least ten minutes. However, the duration of the time interval may be e.g. at least one hour or at least one day. Longer durations provide increased measurement sensitivity, but clearly make the method more time-consuming.
The detector may be installed downhole or above ground, depending on where in the flow system it is desired to monitor the scale formation and/or determine scale characteristics. For example, valve operation can be affected by scaling and so the installation of the detector to monitor scale development at a valve in the flow system may be desirable. A below ground detector may monitor scale development e.g. in the production tubing or in the formation of the hydrocarbon well.
In a second aspect, the present invention provides an apparatus for determining at least one characteristic of scale at a location in a hydrocarbon well flow system, comprising a radiation detector and a signal processor. The radiation detector is adapted to be installed (preferably permanently or semi-permanently) in situ in the system, to obtain (preferably repeatedly) a gamma-ray spectrum from the scale, and to send a measurement signal encoding the spectrum to the signal processor. The signal processor is adapted to receive the measurement signal and configured to analyse spectroscopically the spectrum to determine the abundances of radioactive isotopes in the scale.
Preferably the signal processor is further adapted to determine from the abundances at least one of:
(a) the specific activity of the scale,
(b) the permeability of the scale, and
(c) the amount of radium originally deposited in the scale and thence derive the quantity of scale from the amount of radium and from the relative concentrations of radium and the other scale components in the fluid from which the scale deposits.
The signal processor may be a programmable computer suitably programmed to perform the above-mentioned determinations. Alternatively the processor may comprise dedicated hardware, based on e.g. ROM, for performing the determinations.
The detector may have protective shielding which is adapted, in use, substantially to prevent radiation arriving at the detector from certain directions.
Preferably the detector is adapted to be held stationary in the hydrocarbon well flow system.
The detector may be adapted to be e.g. mounted to a hydrocarbon well production tubing, or cemented to a hydrocarbon well borehole casing.
The apparatus is, therefore, suitable for performing the method of the first aspect of the invention.